The present invention generally relates to semiconductor processing and, more particularly, a system and method to detect defects in a pellicized reticle due to exposure at short wavelengths.
Lithography in semiconductor processing relates generally to the process of transferring patterns which correspond to desired circuit components onto one or more thin films that overlie a substrate. Patterns are transferred from a photomask or reticle onto a photoresist layer which overlies the film on the wafer through an exposure process. If the photomask or reticle contains defects, even submicron in range, such defects may be transferred to a wafer during the exposure. Such defects may be generated by the fabrication process utilized to produce the mask or reticle as well as during subsequent handling and processing. Such defects generally fall into two classes: fatal (or killer) defects and nonfatal defects.
Defects may arise at any stage of semiconductor fabrication, such as during manufacture of a blank reticle or mask or the process steps employed to manufacture a desired reticle.
Inspection tools have been developed and proposed to detect defects in the mask or reticle, such as upon completion of the mask or reticle fabrication sequence. A typical inspection process may examine several characteristics of the mask or reticle, such as linewidth measurements, measurement of the registration among die patterns, determining that all intended features have been transferred to the mask or reticle, and determining if any mask fabrication defects have been produced. Different tools may be utilized for each of these inspections.
By way of particular example, mask fabrication defects are usually located by using transmitted light. The inspection task determines if there is light or no light transmitted at a particular location on a reticle as a function of the intended design. The determination may involve both die-to-die inspections, which involve a visual comparison of two equivalent pattern areas in an array of a die. Any differences are attributable to defects in one or the other inspected regions. A die-to-die inspection, however, cannot detect defects that affect the entire die equally. Accordingly, another type of inspection, called die-to-database inspection, may be utilized. Die-to-database inspection compares an optical image with a simulated image derived from the original design data. This approach, however, requires considerably more image processing power.
After a reticle or mask is determined to sufficiently free from defects a pellicle may be attached to provide protection. The pellicle attachment and/or features of the pellicle itself (e.g., frame films, ventilation, adhesives, etc.), however, may contaminate the reticle as well as generate defects. A pellicle is a membrane that seals off the mask or reticle surface from airborne particulates and other forms of contamination. The membrane is mounted on a metal frame that is attached to the chrome side of the mask or reticle, such that the membrane is suspended above the mask surface. A pellicle also may be mounted to the other surface. While pellicle helps protect the reticle or mask from subsequent contamination, the pellicle and the process of attaching the pellicle provides another place where defects can arise. Accordingly, post-pellicle inspection usually is employed to ensure that no additional defects are caused by the pellicle.
Post-pellicle inspection may be implemented by mounted the mask-pellicle assembly in a projection aligner. Then, the projection aligner is employed to expose round, thin glass wafers that are coated with chrome and resist. The wafers are exposed, developed and etched and stripped to produce an image suitable for inspection by transmitted light. The processed wafers may then be processed on an automatic mask inspection system, such as described above. Defects located on the inspection system that coincide in location on two or more glass wafers are attributable to defects resulting from the application of the pellicle.
Defects further may develop during post-pellicle fabrication, which may include electrostatic discharge (ESD), as well as circumstances associated with reticle storage, the fabrication environment and stepper usage.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
A system and method are provided for detecting latent defects in a mask or reticle, such as to detect defects that may vary as a function of radiation at exposure wavelengths. One aspect of the present invention provides an inspection system utilized in combination with an exposure system. The inspection system is utilized to detect defects in the mask or reticle and store associated inspection data. The exposure system illuminates the mask or reticle with an exposure wavelength, such as to simulate actual exposure experience by the mask or reticle during semiconductor fabrication. Inspection data may be collected for both during and after exposure. A correlation between the inspection data provides an indication of exposure-related defects, which may include defect growth and/or formation of defects caused by the exposure.
According to another aspect of the present invention, the combination of inspection and exposure of a mask or reticle may be implemented with respect to a pellicized mask or reticle. As a result, additional defects, such as transmission degradation, related to use of the pellicle with the mask or reticle under exposure may be detected.
Another aspect of the present invention provides a method of detecting defects in a mask or reticle. The method includes inspecting a mask or reticle and then exposing the mask or reticle to an exposure wavelength. A post-exposure inspection is performed and the inspection information for both before and after exposure is correlated. The correlation provides an indication of defects functionally related to the exposure. As a result of identifying such defects, the occurrence of exposure-related defects during fabrication may be mitigated.